1. Field of the Invention
The present invention relates to an apparatus for the measurement of voltage which measures the voltage of a device such as an IC (integrated circuit) without contact using an electrooptic effect, and specifically to a method of positioning an electrooptic (E-O) probe thereof.
2. Related Background Art
One feature of such an apparatus for the measurement of voltage is having an E-O probe which comprises on electro-optic crystal, the refractive index of which is varied when the voltage is applied. If the voltage is applied to a device to be measured while the E-O probe is close to the device, an electric field in the electro-optic crystal is varied, whereby the refractive index of the crystal is varied. At this point, if light (usually produced in the form of a laser beam) enters into the E-O probe, the state of the light polarization is varied by a change of the refractive index of the crystal in accordance with the applied voltage. Accordingly, if the light which is reflected by a base of the probe and returns is removed by the polarization beam splitter through the wave plate, a change of the applied voltage can be detected as a change in light intensity. Here, a laser source for the measurement may be a continuous wave (CW) source or a pulsed source.
The detecting sensitivity of the apparatus for the measurement of voltage depends on the distance between the E-O probe and the device to be measured. Accordingly, it is important to set the distance accurately. A solution in a conventional case is, for example, to use a double-focus lens as disclosed in:
"J. Appl. Phys. 66 (9), Nov. 1, 1989 pp. 4001-4009", or to use a balance disclosed in the following reference PA1 "IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 41, NO. 3, JUNE 1992, pp. 375-380".
However, in the former conventional case, an E-O probe can be positioned without contact with the device to be measured, but there is one fault: a special double-focus lens is required. Further, in the latter conventional case, in order to set the position of the E-O probe, the E-O probe is brought into contact with the device to be measured once to obtain the reference position, and based on the reference position, the position of the E-O probe is determined. In this case, its structure is very complicated in order to decrease the damage caused by the contact. Specifically, the E-O probe must be fixed at the cylinder which can move up and down in a nearly frictionless state with air blown between the cylinder and an air guide. Their effective mass must be reduced by the balance system. Moreover, the slight vertical movement of the E-O probe is needed for operation by a piezo element fixed at the cylinder, and the position of the E-O probe is required to be read by a position scale installed at the cylinder. Thus, in the latter case, since the peripheral systems of the E-O probe were very complicated and large, the number of electrical pins which provide voltage or signal could not be set around the E-O probe. Further, since the E-O probe is brought into contact with the device to be measured prior to the measurement, there was a danger of breaking the device.